The invention relates to methods and apparatuses for tracking subjects in motion picture sequences, to apparatuses for performing such methods and to related methods and apparatuses. The invention may be applied for example in videophones, or in the digital encoding of general motion picture sequences, for example for publishing motion pictures on optical Compact Discs.
The improvement in the subjective image quality of videophone images that can be achieved by enhancing the area around the user's face, at the expense of the rest of the frame, has been known for some time and successful algorithms to accomplish this task have already been developed. Implementation of such algorithms on commercial hardware, however, has been delayed by the computational complexity. Most successful algorithms rely on techniques for edge detection and operate at pixel rates requiring much high speed hardware.
Some algorithms operate on frame differences for motion detection, a much simpler technique, but they have, however, had problems in the past with changes in overall lighting intensity, movement in the background (a common occurrence in office environments), or even non movement of the user.
Other algorithms exploit the fact that the video compression codec divides the image into blocks (typically of 16.times.16 pixels) for coding, and extracts a few parameters from each. By operating on these much smaller arrays of parameters they can run at a lower rate and are much less demanding computationally.